The present invention relates, in general, to optical coupling of integrated circuits, and more particularly to the transfer of data between electronic devices by means of modulated light beams directed into planar optical waveguides.
As the speed and density of electronic devices increases the delay due to circuit interconnection has become more significant. In the past, interconnections have used multiple conductor cables, often etched on flexible printed circuit board material to reduce the conductor size and spacing to a minimum. This size reduction is limited by the tendency for signals to interact with one another along the length of the cable, called crosstalk. Crosstalk is aggravated by the reduced conductor cross section, reduced conductor spacing and by increased signal speeds. As a result the use of optical interconnect technology has been explored. Several optical interconnect approaches were advanced by Goodman, et al., "Optical Interconnections for VLSI Systems", Proceedings of the IEEE, vol 72, No. 7, July 1984. An example of such an optical interface between integrated circuits is co-pending application by F. V. Richard, Tempe, Ariz., assigned to the same assignee: U.S. application Ser. No. 07/576,914, filed Sep. 4, 1990, now U.S. Pat. No. 5,061,027, entitled "Solder-Bump Attached Optical Interconnect Structure". Further advances in the art have included the use of laminated polymer optical waveguides as the optical medium. This has been described by B. L. Booth, "Low Loss Channel Waveguides in Polymers", IEEE Journal of Lightwave Technology, Vol. 7, No. 10, October 1989. An example of such a laminated polymer optical waveguide used in an optical connector is described in U.S. Pat. No. 4,883,743, "Optical Fiber Connector Assemblies and Methods of Making the Assemblies", issued on Nov. 28, 1989, to B. L. Booth et al, assigned to E. I. Du Pont De Nemours & Co.,Inc., and is incorporated herein by reference. The assignee markets a laminated polymer optical waveguide under the trademark "Polyguide". Laminated polymer optical waveguide such as Polyguide combines the high bandwidth, low crosstalk capability of optical cables with the high density multiple signal capability of electrical cable. In addition this laminated polymer optical waveguide can form simple optical connectors functionally similar to electrical connectors well known in the art.
An optical signal path alone is not useful. To be useful as a path for electrical signals, the optical signals must be interfaced to electrical circuits. Such an interface must meet many requirements. The interface must maintain the high bandwidth, low crosstalk, and high density of the optical cable. The interface must not compromise the capability of the optical cable to form a ribbon comprising a plurality of optical waveguides spaced as densely as 65 parallel waveguides per cm. In addition, the interface must be low cost, must be simple to assemble and to package, and must be compatible both physically and electrically to electronic devices. A simple electrical and optical connector scheme is required. An interface which uses a vertically emitting or sensing optical device such as a vertical cavity surface emitting laser or vertically photosensitive diode rather than a horizontally operating optical device is desirable. Use of a vertically operating optical device aids alignment of the device with the optical waveguide. Typically the outside dimensions of optical devices is not well controlled so some form of adjustment in the vertical plane is required if a horizontally operating optical device is used. In addition such a horizontally operating optical device must be mounted at precisely the same angle as the axis of the optical waveguide, further complicating the assembly process. Finally, it is estimated that a vertical cavity surface emitting laser can be fabricated in high volume for approximately $.25 per device, compared with $1 to $2 for a horizontally functioning equivalent. There is a need for an interface between laminated polymer optical waveguide and electronic devices which meet all of these requirements and uses vertically operating optical devices.